Who uses autopilot in IMC?

[jayemm]

The autopilot in our PA28 failed a few months ago. I first noticed it when, once in the cruise, with the bug and heading set up, I engaged the autopilot and we immediately took a dive to the left.

My 'without thinking' instinct was to use the yoke to recover and was surprised by how hard it was to fight the autopilot. I quickly disengaged the autopilot and recovered. I tried the autopilot again a little later, this time with my hand staying on the switch, with the same effect. On landing I reported the autopilot u/s.

The other day, whilst flying IMC and with the autopilot's imminent return following repair, I wondered what would have happened if the same event occurred whilst in IMC. There's a very good chance that I would not have noticed the turn to the left until it was too late to recover.

ok, so I guess I could always check the Turn Coordinator and AH when engaging the autopilot in IMC, and keep my hand on the switch until everything checks out.

But is it as simple as this?

I've found no 'rules' about the use of autopilot in IMC, and wondered if there are any. What are your 'rules' on this? Do you go as far as to not use autopilot in IMC?

[Final 3 Greens]

Jayemm

Personally I'm not IMC rated, but it mught be helpful to the thread if you define the modes of the autopilot you use, e.g.

- Wing Leveller

- Heading or NAV coupling

- Altitude

As a general rule, I have used PA28 autopilots in VMC to reduce workload when busy and found a wide variation in efficiency.

These days I tend to hand fly and live with the workload.

[Keef]

PA28 autopilots are fine if they are correctly set - but in my experience, they rarely are. I would not rely on ours to get me anywhere except lost. Right now, following the most recent C of A, it steers about 15 degrees right of the heading set on the bug when in "heading" mode, and about 5 degrees right of the OBS in NAV mode.

Before using the autopilot in IMC, check very carefully to see it's doing what it's supposed to. There are also checks to be done on the ground if you plan to use it, although I can't say I've ever seen a PPL doing them.

In IMC, you would be constantly checking heading, height, track, etc anyway so would know immediately if your autopilot was misbehaving - wouldn't you!

;)

[Tinstaafl]

Doesn't matter if it's VMC or IMC. Never engage the autopilot without monitoring its performance.

Applies to any system or device, really.

[2Donkeys]

Engaging the autopilot does not permit you to break your scan if flying under IMC. There seems to be the assumption in the question that once the autopilot is on, the pilot is free to do those other more important things... whatever they might be.

It is worth being aware of some of the other more insidious problems that an autopilot can mask whilst in IMC. Perhaps the best known is the effects of flying in icing conditions. An autopilot with Altitude mode engaged will gradually roll on extra elevator trim as the ice builds up, in order to maintain level flight. The "only" clue to the pilot is the gradual loss of airspeed, or the pre-stall buffet as the a/p runs out of elevator trim. This problem is particularly acute at night, when other visible signs of ice-accretion may not be so obvious.

This example is far from being the only autopilot gotcha in IMC. For this reason, not only should you engage the a/p with caution, you should also disengage with the same caution. It would be a shame to discover that the autopilot had been happily flying along straight and level, but with the aircraft configured for significant nose-down trim... another common fault on GA autopilots.

[jayemm]

Thanks for the feedback so far; it's cautionary advice I haven't had elsewhere.

Our autopilot is Heading only.

[distaff_beancounter]

jayemm I agree with all the above posters.

In addition, I would add my two ha'pence.

I never trust any autopilot, in any light aircraft

Firstly, I always check on the ground, what exactly does this particular autopilot do. For headings, some only follow the heading bug on the DI/HSI, some can be selected to follow NAV1 or NAV2, or the ADF, or the GPS.

Some will also maintain height, but this can be either the actual height when engaged, or the desired height/altitude/flight level can be selected.

Finally, on some of the more seriously expensive light aircraft, the autopilot also controls throttle settings. Rates of climb & descent can also be selected.

Having found out what the autopilot is meant to be able to do, I like to try it out first in VMC, just to check that it is doing, what I think it should.

If I do select autopilot in IMC, then I also monitor it carefully, & deselect it, if it does anything odd!

[GIMPOSH]

War story time hard hats on everyone.

Was once flying a C210 in IMC with the AP engaged in NAV/ALT mode. Without any warning the aircraft violently pitched up and rolled left (roll was much more pronounced than pitch) at max AP rate. Fortunately I was ghosting controls and maintaining scan (hate to think what would’ve happened if I had been head down at night. After nearly S**ting myself I made fast use of the EMERG DISC switch on the yoke.

I tested the AP on the ground after the flight and it functioned normally passing its BIT. I wrote up the US and was told that no fault could be found. People continued to fly the aircraft / use the AP and as far as I know the fault never returned.

To my horror I later found that it had done the same thing to another pilot a few days before my incident but he had decided not to tell anyone about it

I still use the AP in IMC on light AC but I don't trust them like I used to.

[Rod1]

I have a memory of an article on autopilots and certification. If I have remembered this wrong I am sure I will get corrected!

The article said that in order to certify an autopilot it must not be able to put the aircraft into an “extreme” situation for 3 seconds. ” Extreme” was a spin, stall or through VNE.

Since reading this I have reduced my use of the autopilot in IMC to almost zero and I watch everything very carefully!

Rod

[englishal]

Its also advisable to keep a careful eye on the AP in a light aircraft when at any sort of altitude / turbulence. I was flying along at around 11,500' in an Archer III and encountered light turbulence. The aircraft started porpoising with increasing amplitude until I disconnected the AP...I think at lower altitude the AP would've handled it ok, but due to the thinner air it started getting itself a bit confused.

EA

[Final 3 Greens]

EnglishAL

I'm not so sure that its purely altitude related....

A few years ago I flew a shiny new Archer III on a convective day in mid summer and set the a/p to altitude hold mode at 3000'.

An oscillation started and the only way I could stop it was to disengage and apply some homo sapiens induced damping.

To be fair it was a pretty bumpy day, but my impression was that the a/p was "hunting" and thus increasng the severity of the porpoising.